Position sensors are used to provide inputs for a variety of electronic devices. Some of these sensors are electromechanical devices, such as the ubiquitous "mouse" that is used to provide position input signals to digital computers. Other sensors, which are non-mechanical, usually make use of electrostatic or magnetic fields to provide position information. An example of an electrostatic sensor is a capacitive button switch, which is actuated when the user places a finger thereon; in so doing the user effectively increases the capacitance of a capacitor, with the resulting increase in capacitive current being sensed to indicate actuation of the button.
The non-mechanical sensors are advantageous in that they have no moving parts and moreover are, in theory at least, not restricted to operation over a small area such as a mousepad or the like. Actually, however, because of configuration and sensitivity considerations, these sensors are limited to a small area; indeed, when they are used as "pushbuttons," this is a desirable attribute of capacitive sensors.
Electromechanical sensors are limited by their construction to detection of specific types of user movements. For example, a mouse can detect position along a two-dimensional surface and transmit the user's actuation of "click" buttons mounted on the mouse; three-dimensional location and gestures other than the familiar button click, however, are beyond the mouse's capacity to detect. The prior electrostatic and magnetic sensors suffer from the same disabilities.
In fact, determining the position, mass distribution or orientation of an object within a defined space represents a highly complex problem due to the difficulty of resolving among cases which, while physically different, produce identical or insubstantially different sensor readings. For example, in an electric-field sensing system, a large object far away may produce the same signal as a smaller object close by. Naturally, the more sensors one employs, the greater will be the number of cases that may be unambiguously resolved, but as yet there exists no methodology for systematically designing a sensor arrangement capable of resolving a desired set of cases with the fewest number of sensors. Indeed, no current electrostatic sensor arrangement is capable of providing three-dimensional information throughout a defined space.
For example, a more advanced version of the capacitive button switch is described in published PCT application WO 90/16045 (Tait), which describes an array of receiver electrodes arranged about a central transmitting electrode. Even this type of configuration, however, is relatively crude in terms of the information it provides, since what is measured is variation in weighting among the arrayed electrodes. Arrangements such as this do not provide three-dimensional positional information. They do not meaningfully reduce the number of devices (i.e., electrodes) necessary to characterize position and orientation, nor provide an approach to obtaining an optimum number of devices. Moreover, the Tait device is not employed in a manner that is even capable of operating over three dimensions, much less distinguishing among different orientation/position cases. It is expected that contact will be complete in all cases--that is, the user's finger will actually touch the transmitting and receiving electrodes--rendering the approach unsuitable where such contact is not possible.